Agar plates experiment: microbes on books

Two books about microbes: Carl Zimmer’s Microcosmand Ed Yong’s I Contain Multitudes. I could do a normal comparative book reviews, or…. I can check which one has the most microbes growing on it, and make another elaborate home science experiment video about it.

Check the video below to find out which book had more microbes, and scroll down to find out how I made the agar plates.

What are agar plates?

Agar is a gelatin-like material produced from algae. It dissolves in hot water, but turns into a hard jelly when it cools down. Biologists use it to create a solid growth medium for bacteria. You can grow bacteria in liquid with nutrients, but if you want to see or study separate “colonies” (groups of bacteria that grow from one individual bacterial cell) then you can use an agar plate made out of the same nutrient-rich growth medium. They’re simple to make. So simple, you can even do it at home!

Making agar plates

I started with a recipe from The Kitchen Pantry Scientist. However, instead of beef bouillon (stock) cubes, I used vegetable stock cubes. It worked, but left me with some green bit in my plates from the vegetables! The stock is used as food for the bacteria to grow in, so I wasn’t sure whether vegetable broth would work as well as beef broth, but it seemed to be okay. (Perhaps if I had used beef stock I’d have more microbe colonies, or different species would have flourished, but I’m happy with just seeing anything grow.)

I then brought it to a boil, and turned the heat down. You only need to boil it very briefly. The purpose of heating is to sterilize the broth and to make sure all the agar is dissolved. If you boil it too long, you just end up with dark plates because you lost too much water and/or the sugar burned.

Growing bacteria

In a lab, you would put the agar plates in a warm incubator to help them grow. I didn’t have that at home, so they just sat on a shelf of my bookcase for a few days. Colonies started appearing the next day, but I waited a bit longer for them to get bigger. In an incubator, it would have all gone a bit faster.

Eva Amsen is a writer, science communicator and blogger. She has been writing about science and scientists in art/culture/life since 2005, both on this blog and for other sites and publications.
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